Recently, complex flexible packaging is developed by industry players due to package strength requirement, food protection needs, labeling and appearance needs of package. The complex flexible packaging is prepared via the laminated films produced by so-called laminating adhesives (also called laminate adhesives). The current main stream of such a laminate adhesive is a two-component (or two-part) type polyurethane (PU) adhesive, composed of a first component base resin having an isocyanate (NCO) group (“First Component”) and a second component curing agent (also called hardener) having one or more active hydrogen groups (“Second Component”). The two-part type polyurethane adhesive has excellent adhesivity, durability, and heat resistance properties. Further, it can be widely applied in the manufacture of film/film and film/metal foil laminates including multilayer laminates in food packaging industry and other industries.
Typically, the first component contains an isocyanate-containing polyurethane prepolymer obtained by the reaction of excess diisocyanate with a polyether and/or polyester containing two or more active hydrogen groups per molecule. The second component usually is a polyether and/or polyester functionalized with two or more hydroxyl groups or the like per molecule. The two components are combined in a predetermined ratio and applied on one of the film or metal foil substrates, which is then laminated to a second substrate. Application may be from a solution in a suitable solvent using gravure or smooth roll coating cylinders or from a solvent-free state using special application machinery.
Solvent-free or solventless laminating adhesives, that can be applied at 100% solids and that do not contain either organic solvents or water, have a distinct advantage because they can be applied and operated at very high line speed. This is due to the fact that there is no organic solvent or water which must be removed from the adhesive. The application speed of solvent or waterbased laminating adhesives is limited because the solvent or water must be effectively dried in an oven, or otherwise removed, and such solvent/water removal process is generally time-consuming.
In order to obtain NCO-terminated PU prepolymers, it is a common practice to react polyhydric alcohols with an excess of monomeric polyisocyanates—generally at least predominantly diisocyanates.
It is known that, irrespective of the reaction time, a certain quantity of the polyisocyanate used in excess reamains unreacted. The presence of monomeric polyisocyanate is problematical, for example, when readily volatile diisocyanates have been used as the monomeric polyisocyanate. “Readily volatile” as used herein represents the property of the substances having a vapor pressure of more than about 0.0007 mm Hg at 30° C. or at a boiling point less than about 190° C. (70 mPa).
Even at room temperature, the significant vapor pressure of volatile diisocyanates, such as 2,4-toluene diisocyanate (TDI), is a serious issue during spray application process. Since isocyanate vapor is toxic, the use of products with a high content of readily volatile diisocyanates involves elaborate measures on the part of the users to protect themselves, more particularly elaborate measures for keeping the surrounding air fit to inhale, as legally stipulated by the maximum permitted concentration of working materials as gas, vapor or particulate matter in the air at the workplace.
Because protective and cleaning measures generally involve considerable financial investment and operating cost, there is a need for products which, depending on the isocyanate used, have a low content of readily volatile diisocyanates.
Currently, the vast majority of adhesives marketed are based on petroleum derived components. The use of petrochemicals such as polyester or polyether polyols is disadvantageous for a variety of reasons. Because petrochemicals are ultimately derived from petroleum, the production of a polyol requires a great deal of energy, as oil must be drilled, refined, and processed to make the polyols. Moreover, petrochemical based polyols are subject to price fluctuations and availabilities of petroleum and natural gas. As the public becomes more aware of environmental issues, there are distinct marketing disadvantages in using petrochemical based products. With the continuously growing demand for “greener” products from consumers, there is a motivation to replace polyester or polyether polyols, as used in the production of polyurethane adhesives, with a more versatile, renewable, less costly, and more environmentally friendly component.
Efforts have been made to address these challenges. Bio-based oil sources such as soy oil, castor oil, palm oil, linseed oil, or chemicals derived from them for instance oleochemical polyols, are suggested as substitutes for the polyols in the second components of the urethane reactants, for manufacturing foams, elastomers, plastics, adhesives and other products, as they provide the promise of a relatively stable and renewable raw material source.
U.S. Pat. Nos. 6,624,244 and 6,465,569 disclose the combination of a multi functional polyol such as ethylene glycol and blown soy oil with an isocyanate to form urethane foam. U.S. Pat. No. 6,649,667 discloses the reaction of a mixture of polyester or polyether Polyol and a blown vegetable oil with an isocyanate in a reaction-injection-molded (RIM) process to produce closed cell polyurethane foam. U.S. Pat. No. 6,180,686 discloses polyurethane foam prepared by reaction of an isocyanate with a mixture of a blown vegetable oil and a polyether polyol. Because soybeans or the like are renewable, relatively inexpensive, versatile, and environmentally friendly, they are desirable as ingredients for foam manufacturers. Soybeans may be processed to yield fatty acid triglyceride rich soy oil and protein rich soy flour.
DE 44 01 572 A1 describes a two-component polyurethane adhesive based on an isocyanate component and on a polyol component, which contain an oleochemical polyol such as castor oil from 2 to 7 percent by weight, based on the oleochemical polyol, of at least one di- and/or trifunctional alcohol, the hydroxyl value of the alcohols or the mixtures thereof being 1,100 to 1,850. This adhesive is suitable for non-load bearing wooden structural members.
U.S. Pat. Publication No. 20040138402A1 discloses a two-component polyurethane adhesive for wooden materials comprising components A and B wherein A) a polyol mixture, containing at least a) 10 to 98 wt. % of at least one oleochemical polyol, b) 1 to 7.5 wt. % of at least one diol having a hydroxyl value of 400 to 2,000 and c) 1 to 7.5 wt. % of at least one tri-, tetra- or pentafunctional polyol having a hydroxyl value of 200 to 2,000, the wt. % of each of a), b) and c) based on the whole of the polyol mixture, and B) at least one polyisocyanate, wherein the NCO/OH ratio of components A) and B) is within the range of 1.5 to 0.9, and further comprising from 0 to 85 wt. %, based on the total weight of the adhesive, of at least one auxiliary substance; wherein the polyol mixture contains up to 60 wt. %, based on the total weight of the polyol mixture, of a resin homogeneously dissolved therein. The adhesive is useful for bonding of load bearing wooden building components.
U.S. Pat. Publication No. 20060276614A1 describes a bio-based multipurpose urethane adhesive in which the B side component is derived mainly from a vegetable oil, preferably soy oil. Preferably the vegetable oil derivative is a hydroxylated vegetable oil which is present in the B side component at a concentration from 65 to 98 weight percent. Optionally, a phosphated soy oil can be present in an amount from 0.1 to 15 weight percent to enhance the adhesive properties, particularly for use on metals. Also, water can be present in an amount from 0.1 to 10 weight percent with a slight amount of a surfactant to stabilize the mixture. Catalysts are included in the B side component to reduce the amount of the A side component and to accelerate curing of the adhesive. The A and B side components are admixed in amounts which provide from 40 to 90 weight percent of bio-based content in the adhesive.
CN101280167A discloses a solvent-free polyurethane laminating adhesive that could be processed at room temperature, comprises two components, where the hardener comprises 1%-10% weight percent of polyol a) with carbon chain length less than 12 carbon, at least two functionality 30% to 70% of polyol b) 20% to 70% of polyol c) modified by plant oil, and 0.01% to 1% of curing speed adjusting agent. The base resin comprises polyurethane prepolymers obtained by the reaction of 25% to 50% of polyol modified by plant oil or other polyester polyol or polyether polyol and 50% to 75% of diisocyanate.
Primary Aromatic Amine and Hazardous Effect:
In order to reduce the volatile effects of chemicals used in the laminating process, low-volatility diisocyanates, such as diphenylmethane diisocyanates (MDI), are typically used rather than high volatile diisocyanates TDI, wholly or partly. But the PU prepolymers based thereon have viscosities that are outside the range relevant to simple methods of application. Consequently, by adding an excess of diisocyanate monomers as reactive diluents, the viscosity of the polyurethane prepolymers could be reduced. Nevertheless, the incomplete reaction of the reactive diluents is capable of “migrating,” even into the bonded materials. By contact with moisture (H2O), the isocyanate groups of migrated material may react to form primary undesirable aromatic amines (R—N═C═O+H2O→R—NH2+CO2).
Migrated material and side reaction are undesirable in the food packaging industry, because the passage of the migrated material through the packaging material can lead to contamination of the packaged food product. To avoid contamination, the laminates must be “migrate-free”. Another unwanted side effect caused by the migration is the antisealing effect during the production of bags or carrier bags using laminated plastic film. Thus, substances and preparations containing, for example, more than 0.1% free TDI fall under hazardous material regulations and have to be identified accordingly. The obligation to do so involves special measures for the food packaging industry.
Accordingly, reactive adhesives that are suitable for the production of composite materials should have a suitable application viscosity, but not contain or release any volatile or migratable substances into the environment or packaged material as such could cause hazardous effects to human beings. The current invention addresses these issues.